Did the ice cream seller say why, for not selling to them?

Prohibition has never worked in human history.

America did it, and all that happened was the rise of illicit liquor sales and bootleggers smuggling alcohol from across Canada and the rise of underground distilleries (speakeasies).

Humans will always look for ways to satisfy their cravings no matter what. If you ban something, they will just look for another addiction to fill up that hole instead.